It was shown previously that two core proteins of Rauscher leukemia virus, reverse transcriptase (RT) and p30 interact with each other and form a complex with enhanced DNA synthesis. When, however, RT was interacted with another core protein p12, two molecular size of complexes can be seen: one with high molecular weight and another low molecular weight. High molecular weight RT-p12 does not interact with p30. In contrast, low molecular RT-p12 interacts with p30 and forms a large molecular weight complex RT-p12-p30. Such a complex is capable of increasing the synthesis of DNA with a greater fidelity than RT alone. We now have included p10 in our studies. We hypothesize that low molecular weight RT-p12 may be a building block that, upon binding p30 and p10, forms a large DNA synthesizing apparatus.